Background: Left ventricular diastolic dysfunction (LVDD) is associated with chronically elevated left ventricular filling pressures that promote left atrial (LA) remodeling and increase the risk of atrial fibrillation (AF). Because AF is frequently asymptomatic during its early stages, identifying echocardiographic markers that facilitate early risk stratification is of considerable clinical importance. This study investigated the association between LA enlargement and AF in elderly patients with LVDD using conventional echocardiographic LA size parameters. Methods: This prospective observational study enrolled 100 consecutive patients aged ≥ 60 years with echocardiographically confirmed LVDD and no documented history of AF between December 2023 and May 2024. Comprehensive transthoracic echocardiography was performed to assess LA diameter, biplane LA volume using Simpson’s method, LA volume by the area–length method, and indexed LA volume (LAVI). The presence of AF was determined by standard 12-lead electrocardiography and rhythm assessment. Echocardiographic parameters were compared between patients with and without AF using appropriate statistical analyses. Results: The study population had a mean age of 68.37 ± 6.31 years, and 52% were male. AF was identified in 15% of participants, while mild LA enlargement was observed in 18%. Among patients with LA enlargement, 72.2% had AF compared with only 2.4% of patients with a normal LA size (p < 0.001). Patients with AF demonstrated significantly greater biplane LA volume (69.10 ± 9.61 vs. 52.21 ± 6.28 mL), LA volume measured by the area–length method (71.42 ± 9.51 vs. 54.03 ± 6.53 mL), and indexed LA volume (37.33 ± 3.81 vs. 28.87 ± 3.65 mL/m²) than those without AF (all p < 0.001). Conclusions: Left atrial enlargement measured using echocardiographic assessment, particularly indexed LA volume, is strongly associated with atrial fibrillation in elderly patients with left ventricular diastolic dysfunction.